1. Field of the Invention
The present invention relates to a method for fabricating semiconductor devices, and more particularly to a technique for forming a smooth under electrode with a tight structure in order to form a dielectric film exhibiting a high dielectric constant in the fabrication of capacitors for highly integrated semiconductor devices.
2. Description of the Prior Art
Typically, electrodes of dielectric elements are employed in DRAM's and non-volatile FRAM's as under electrodes for dielectric thin films for the charge storage. They are also used as under electrodes for thin films of display elements in thin film infrared sensors, optical memories, optical switches and optical modulators.
In the case of DRAM's which use such under electrodes and include unit cells each consisting of one MOS transistor and one capacitor, the capacitance C of the capacitor can be expressed by the following equation: EQU C=(Eo.times.Er.times.A)/T
where, Eo represents the dielectric constant in vacuum, Er the dielectric constant of the dielectric film, A the area of the capacitor, and T the thickness of the dielectric film.
As understood from the equation relating to the capacitance C, the capacitance is proportional to the surface area of the storage electrode. In this regard, it is difficult to obtain a sufficient capacitance in highly integrated semiconductor devices because the high integration of semiconductor devices inevitably involves a reduction in cell dimension, thereby reducing the surface area of the storage electrode to which the capacitance is proportional. In the case of DRAM devices, it is important to reduce the cell dimension and yet obtain a high capacitance of the capacitor, for the high integration. As apparent from the above equation, the capacitance C can be increased by forming the dielectric film using a dielectric material exhibiting a high dielectric constant Er while reducing the thickness T of the dielectric film. In this case, however, the surface morphology of an under electrode disposed beneath the dielectric film serves as an important factor in the fabrication of capacitors. For example, where the under electrode has a rough surface morphology, namely, hillocks and pin holes at its surface, a degradation in electrical characteristic such as a short circuit may occur due to the hillocks and pin holes. Such a phenomenon results in a degradation in the stability and uniformity of capacitors. As a result, it is impossible to achieve an improvement in the reliability and productivity of semiconductor devices as well as a high integration of such semiconductor devices.